In a currently-used apparatus that performs printing onto a long medium, a print format has been switched from a transaction printing to POD (Print on Demand) printing. The transaction printing performs typical printing onto continuous-form paper. The POD printing often performs printing depending on a print medium such as exclusive paper for inkjet or coated paper. The POD printing requires printing with higher accuracy.
Some of inkjet printing apparatus include one provided with a recording unit. The recording unit has inkjet heads with nozzles in staggered arrangement for compatibility of high speed recording and high resolution. Such an apparatus may generate printed results as under. Now reference is made to FIGS. 3 to 5. FIG. 3 is a schematic view illustrating a printed result with neither skew nor serpentine. FIG. 4 is a schematic view illustrating a printed result with skew. FIG. 5 is a schematic view illustrating a printed result with serpentine.
When the printing sheet contains neither skew nor serpentine as illustrated in FIG. 3, a printing region with a given area at a certain density has no defective part and thus has only a normal part. On the other hand, when the printing sheet is skewed as illustrated in FIG. 4, a defective part is generated. The defective part includes white lines caused by non-printable portions due to clearances of the inkjet heads in the staggered arrangement, and black lines caused by overlapped nozzles of the inkjet heads in the staggered arrangement in a transportation direction. Moreover, when the printing sheet is serpentine as illustrated in FIG. 5, the following detective parts are generated. That is, the defective parts include a defective part with white and black lines that are the same as that in the skewed printing sheet, a normal part, and a defective part in which the white and black lines are transposed due to serpentine in an opposite direction.
Consequently, the following has been suggested. See, for example, Japanese Unexamined Patent Publication No. 2002-46912A. That is, transport rollers transport a long print medium, and a skew correcting mechanism corrects skew of the print medium. Thereafter, an image generator disposed downstream of the skew correcting mechanism generates an image on the print medium.
However, the conventional example with such a configuration has the following drawbacks.
That is, the currently-used apparatus includes the skew correcting mechanism. Consequently, this suppresses skew of the printing sheet, and thus a printing defect caused by the skew are unlikely to occur. On the other hand, the skew correcting mechanism has no function of attenuating serpentine. This causes a printing defect caused by the serpentine. Accordingly, two transport rollers are combined to generate an S-shaped pass that allows serpentine attenuation, and the S-shaped pass is disposed downstream of the skew correcting mechanism. However, such a configuration may cause the following drawback. That is, large skew may occur depending on parallelism of the two transport rollers of the S-shaped pass, leading to possibility of the printing defect caused by the skew.
Other types of apparatus except for the printing apparatus may possess the drawback of the skew and/or serpentine generated upon transportation of the long medium.